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Thirteen people were killed and 31 injured after a tour bus slammed into a semi-truck in the Palm Springs area Sunday, Oct. 23, 2016. (Published Sunday, Oct. 23, 2016)

In a horrific early morning crash, a tour bus slammed into the back of a truck on Interstate 10 north of Palm Springs before sunrise Sunday, killing 13 and injuring 31, authorities said.

The Riverside County Coroner's office confirmed 13 people aboard the bus were killed, including the driver, in the disastrous crash that shut down the entire westbound 10 Freeway west of Indian Canyon Road. Traffic was being diverted off the freeway at the Indian Avenue offramp. The freeway reopened around 4:30 p.m., according to the California Highway Patrol.

Rescuers continued to search the wreckage for more victims in the early stages of the crash. The first images from the scene showed firefighters using ladders to get into the passenger compartment of the bus, which had been peeled back from the vehicle's undercarriage about one third of its length.

The front of the bus was demolished as the back of the semi was shredded from the monstrous force of the crash.

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It wasn't immediately clear what caused the collision, but the tour bus was traveling much faster than the semi, CHP Chief Jim Abele said at a news conference.

The trailer of the semi was lodged 15 feet into the front of the bus, Abele said.

The bus, owned by LA-based USA Holliday Bus, had departed the Red Earth Casino in Salton City and was en route to a location in Los Angeles when the crash was reported to CHP at 5:17 a.m.

The CHP said there was no indication that the bus slammed on the brakes before plowing into the back of the semi-truck, leading investigators to believe fatigue may have been a factor. They were not ruling out some type of medical emergency on the driver's behalf, such as a heart attack.

The driver, along with the deceased passengers, were not yet identified to the public.

It appeared that all passengers aboard had been asleep at the time of impact. While they had likely all been seated properly, many ended up in the front end of the bus due to the force of impact, Abele said.

CHP reported that 31 people had been injured. Desert Regional Medical Center, Eisenhower Medical Center and JFK Memorial Hospital all received and treated patients from the crash, ranging from five critical patients to others with minor injuries.

A trauma surgeon said the injuries included facial trauma but few broken bones, which is unusual for a high-speed vehicle wreck.

Dr. Ricard Townsend of Desert Regional Medical Center in Palm Springs told reporters Sunday that the bus struck the collapsible trailer of a semi-truck, sending many of the likely unrestrained passengers flying through the air.

He called the widespread facial injuries a hallmark of those not wearing seat belts. He says the injuries indicate the bus was slowing down at the point of impact.

Because the investigation was still in its early stages, not all of the victims had been identified. CHP encouraged family members to reach out if they had not heard from their loved ones.

Family members were directed to the Riverside County Fire Department facility at 800 S. Redlands Ave. in Perris, or to call (951) 443-2300. The fire department was setting up a crisis response center there.